Magnetic Board... "Don't ask for "Sheet Metal" because that is super expensive...some were $40 a sheet. Go to the metal chimney or roof venting section. The pieces came in 2 sizes. This one was the largest 24" x 36" and they were only $5.00 and $7.00 each."
Homasote. This is technically a compressed fiberboard product that is sometimes used for soundproofing and insulation. Great for large bulletin boards because it takes pins really easily, you can cover it with fabric, paint it, or frame it. It's about $12 for a 4 foot x 8 foot sheet. - Cheaper than cork sheets at Hobby Lobby.